Underwater Caribbean

Underwater Caribbean

I look over Bill's shoulder to see what he's pointing at. "Fingerprint cyphoma, very rare," he writes on his slate. I swim around him for a better look, tipping my head down and fins up, poking one finger into the sand for stability. Finally, I see it. An inch-long orange-and-black creature clings to a branch of coral.

The fingerprint cyphoma is a close cousin to the flamingo tongue. They're both carnivorous snails. They both feed on soft corals. They both have a hard pale shell, which is usually covered with a brightly patterned, retractable mantle of flesh. To the human eye, they look identical, except for the pattern on the mantle. The spots on a fingerprint cyphoma are large and densely striped, while the flamingo tongue's are smaller and solid orange. There's another difference: In the Caribbean, only one fingerprint cyphoma is found for every 10,000 flamingo tongues.

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I wonder why two creatures so identical in size, behavior, and habitat have such different populations, but I can only surmise that they're built differently in ways I can't see. One might be choosier about food than the other or less adaptable to different temperatures. When I go looking for scientific information, I learn that the rare fingerprint cyphoma does more damage when it feeds than the common flamingo tongue, consuming a greater mass of coral and leaving the host animal more deeply wounded.

Good thing for the coral, then, that there aren't too many fingerprint cyphomas around. But this tidbit just opens up more questions. Are gluttony and rarity connected? Alone, the plants and animals that make up an ecosystem are dumb, but in aggregate, the system just might be brilliant. It seems smart enough, anyway, to make sure that a rapacious creature can exist only in limited numbers.

It's my last day underwater, and everything I learn opens up new questions. Indeed, much of marine science seems to be terra incognita: As some marine species face population plunges and others become extinct, the Census of Marine Life, a 10-year global project, says the pace of discovery of new species is accelerating. It has some 75,000 species in its database now. In 2006 it discovered, among many new species, creatures that live in deep-sea hydrothermal vents, others that dwell under nearly half a mile of Antarctic ice, and a shrimp thought to be extinct for 50 million years.

During the interlude, I bask in my full-length black wet suit, storing up heat for the next plunge and wondering if the forward starboard bench is long enough for a nap. I've been sleeping unusually well this week, which I initially attributed to a healthy combination of physical exertion and mental relaxation. Another diver reminded me, though, that it's probably the result of the nitrogen we absorb into our bloodstreams at depth.

While I contemplate a nitrogen nap, everyone around me is abuzz. Jeannette, who has a Ph.D. in ichthyology, talks with childlike wonder about a clutch of squid she saw just before surfacing. "They just hung there looking at me, and I looked at them," she says. Blue and Cassandra are standing in the stern, talking with Bill about what they saw underwater. Whether it's the excitement of the moment, or because he has actually grown to like us, Bill seems to be in a good mood, jerking his arms like a marionette as he describes a frogfish. He does more than 400 dives some years, but he hasn't lost his passion for the underwater world.

Our guessing game is on everyone's minds. As our species count for the week climbed above 200, low-guessers were eliminated, but many of us are still in the running. And the species bet has been superceded by an even more important competition. On the last REEF trip to St. Vincent, the surveyors counted 227 species in a week. As of last night, we had reached 221. We won't be able to hold our heads high if we don't beat them.

I remember that I can't nap anyway, because I have to study for a test I'm taking later this afternoon. REEF categorizes volunteers into Levels 1 through 5. At 4 and 5, you're considered "expert" and offered the tantalizing possibility of subsidized dive trips. But I'm uncertain of even graduating to Level 2, which will require me to score 80 percent on an identification test of 25 common species. I've persuaded English Steve, the other beginner, to take the exam with me. While I express optimism about our prospects, he speaks of certain failure, a transatlantic cultural difference. On deck, another diver, Bob, holds up my underwater Reef Fish and quizzes me.

After dive No. 2, we—or rather, some of the six divers on our boat not including me—have racked up four new species, in part because Bill made a point of taking us to a mucky, critter-filled site called Chandler's Nursery. But we don't know what the other boat has seen today, and in any case, numbers aren't official until diligent Mike, our designated species-totaller, makes his evening announcement. But we're so close now, we can almost taste victory.

After our last speedboat jaunt along the shore, a quick lunch, and more cramming, I sit down with Steve and Warren (who isn't really a beginner) to take the test.

We all pass with 90 percent. I start plotting future fish counting in Bonaire, Cozumel, and the Turks and Caicos. Level 3, here I come.

Our species total for the week, announced by Mike at sunset at a hilltop restaurant overlooking the sea, is 229. Franklin, a retired schoolteacher, guessed closest at 221, but I realize at this point that no prize was ever specified. It seems that he wins mainly glory. More important, we beat the last group of surveyors by two species.

The next evening, the last for most of us on St. Vincent, Bill takes us all out for drinks on Young Island, the luxury resort we've been gazing at all week, just across a shallow channel from our own hotel and Dive St. Vincent. After a two-minute ride by shuttle boat, we're all sitting on a new veranda, sipping sweet fruity cocktails and looking back at where we've been. And talking about slender filefish.

We finally say goodbye to Bill, the men with handshakes and the women with hugs, cheek pressed to scratchy cheek. As I turn to go, Bill reaches into his pocket and pulls out a small, hard, smooth-as-glass object, which he hands to me. It's the shell of a flamingo tongue, white on top and pale orange, as though stained, where its mantle used to sit. It may not be 1-in-10,000, but it's coming from him, so I slip it into my pocket like a piece of deep-sea treasure.

Elisabeth Eaves is a staff writer at Forbes.com and the author ofBare.